Hydrocarbon wells may be abandoned for a variety of reasons, such as when dry, when no longer economical to produce, or well integrity has been compromised in some way. It is common practice to plug the well before abandoning it, e.g. to prevent seepage of hydrocarbon product from the well. This can also apply to water injectors, i.e. bores which have been drilled in order to pump water into a reservoir to increase bottom hole pressure.
It is also possible to abandon only part of a well. One cost effective way to enhance production is to permanently abandon the bottom of the well, but use the existing slot to sidetrack the well to reach new payzones. The cost can often be cut in half when sidetracking an existing well instead of drilling a new horizontal well. This process is known as “slot recovery.”
Plugging can also be temporary, e.g., to allow for workover, a long shut-in, or for converting an exploratory well to a production well. The Norwegian shelf's competitive position, or NORSOK, standards state that the integrity of materials used for temporary abandonment should be ensured for the planned abandonment period times two.
In oilfield jargon “plug and abandon” or “P&A” refers to preparing a well to be closed permanently (or at least until prices or technology developments warrant reentry). The earliest oil wells were abandoned without any plugging, but the first plugging requirements were enacted by Pennsylvania in the 1890s. However, prior to modern regulations set in the '50s, many wells were abandoned with plugs consisting of brush, wood, paper sacks, linen or any other material that could be pushed into a well to form a basis for the dumping of one or two sacks of cement to “plug” the well. Current procedures are significantly more disciplined however.
Commonly, plugging may be achieved by injecting a settable substance or medium, e.g. cement, into the well. A well will normally have production perforations, that is to say apertures in a well liner or casing through which hydrocarbon product enters from the rock formation and travels to the surface. During plug and abandonment operations it is common to seal (“squeeze”) production perforations with cement or another settable medium, which may then form a permanent barrier to lateral flow across the perforations and in/out of the well.
Other reasons exist to plug or seal a well. For example, one can seal leaks in a well, such as casing leaks. Parts of a well can be shut down for production or fracking, and the production or fracking moved to a different zone. Perforations can also be sealed for water shut off. Thus, there are many reasons for plugging a well or for sealing a casing or a portion thereof.
Usually cement is used for plugging wells or sealing casing, but cement can be less than satisfactory as it has a tendency to mix with drilling mud or other downhole fluids and this can cause early failure if the fluids cause channeling in the cement. Additionally, cement can be difficult to pump through perforations in joints, and the cement plugs may not suffice if the formation is weak. Therefore, in some cases, metal is used in place of cement for plugging and sealing operations.
US20060144591, for example, describes a method and tool for applying a fluid seal in a well structure by putting meltable repair material (e.g, eutectic metals or solder) and then ignite exothermic reactant material to melt the meltable material. The molten mass then flows into the defect and supposedly seal the perforations. However, the disclosed device only uses thermite sporadically, and positioning of the device and plugging material are not customizable or accurate enough to reduce the overall operation cost and time. The device fails to be efficiently retrieved and reused, especially when the wells need repeat treatment if not sealed conforming to legal requirements.
Another heater tool that uses thermite is commercially available from BiSN Oil Tools, and described in WO2011151271 and WO2014096858. The BiSN Wel-lok M2M Bridge Plug™ can run on standard wireline, slick line or coil tubing. It uses a bismuth-based alloy that is melted in situ using a chemical reaction (thermite) heater. However, this tool can only heat a small volume of alloy at a time. Thus, its applications are limited.
WO2014108431 discloses a method for plugging a hydrocarbon well, including expanding a section of a tubular within the well to contact the casing lining of the well. The expanded sections are then melted to form a plug for the portions to be plugged. However, it is unclear how the molten material would flow horizontally into the holes when there is no support from the bottom to push the molten material up and force it to flow laterally.
U.S. Pat. No. 6,474,414, entitled “Plug For Tubulars” is directed to the use of molten solder for providing a plug in a subterranean well, which may be poured or otherwise applied directly upon a platform for the molten solder in the well.
U.S. Pat. No. 9,181,775, entitled “Sealing method and apparatus” is directed to the use of a mandrel having multiple rings that are spaced apart to allow molten material solidify between the rings to form a plug. However, this design does not facilitate lateral filling of the molten material in perforations and leaks.
Therefore, there is still the need for a plugging device or tool that is both efficient for plugging and sealing operations by forcing the molten metal plugging material flowing laterally into the perforations and having a ignitor that can be reconditioned and reused.